IMO you should take that as proof your particular batteries have 12.2v as their 50%. You should not stick with that 12.1v idea. Actually some Wet batts do use 12.2. Trojan uses 12.1, AGMs have another.
In any case the "morning voltage" (with nothing much on and before solar kicks in) would be used to decide if you need a recharge that day or can wait till next day. You might well have to recharge at 12.3
You note your voltage rate of decline morning by morning from arrival. Starts at 12.7 but first morning it is (example) 12.5 Next morning it is 12.3 So you say "Hmmm, tomorrow it will be 12.1"
Now you have to decide to either recharge today from 12.3 or if you are due to be away today, how much harm can you do by waiting till it is 12.1? You know it is ok to go below 50% a few times, just not every time. So it is down to how many times have you gone below so far.
Also you can say you are here to have fun so you are going to keep to your planned outing and too bad if the batteries take a small hit. Who is running this place anyway, me or the batteries? :)
